Trehalose is a widespread disaccharide, occurring in bacteria, fungi, insects and plants. In microbes, trehalose accumulation is generally associated with stress resistance, not at least with desiccation and osmotic stress resistance. In plants, however, except for some resurrection plants such as Selaginella lepidophylla, the role of trehalose is less clear.
In most cases, trehalose synthesis is a two-step process in which trehalose-6-phosphate synthase (TPS) synthesizes trehalose-6-phosphate (T6P), followed by a dephosphorylation to trehalose by T6P phosphatase (TPP). Although in most plants trehalose is hardly detectable, multiple homologues of both TPS and TPP genes are present, e.g., in Arabidopsis (Vogel et al., 2001; Leyman et al., 2001; Eastmond et al., 2003). Trehalose accumulation obtained in transgenic plants, transformed with heterologous trehalose biosynthesis genes, leads to an improved abiotic stress tolerance (Garg et al., 2002; Jang et al., 2003). However, the absence of significant trehalose accumulation in most plants, in spite of the presence of multiple trehalose biosynthesis genes, argues for a regulatory role of the gene products, rather than for a role of trehalose as stress protectant. Indeed, several authors suggest a regulatory role for TPS (Avonce et al., 2004) and its gene product T6P in sugar metabolism (Eastmond et al., 2003) and starch synthesis (Kolbe et al., 2005). T6P is indispensable for carbohydrate utilization and growth (Schluepmann et al., 2003), but accumulation of T6P seems to cause growth inhibition in seedlings (Schluepmann et al., 2004). The present data are sometimes conflicting and the role of the trehalose biosynthesis genes is still far from clear. None of these publications makes a link with a possible role of plant TPS, in particular class II plant TPS, in plant growth and yield.
EP0901527 discloses the regulation of plant metabolism by modifying the level of T6P. More specifically, they claim an increase in the yield of plants by increasing the intracellular availability of trehalose-6-phosphate. However, rather conflicting, they also claim the stimulation of growth of a plant cell or tissue by decreasing the intracellular availability of trehalose-6-phosphate. Again, as shown in the recent literature, this is indicating that the T6P balance is very delicate and far from straightforward. The inventors realized the modulation of the T6P content by expressing heterologous TPS and TPP genes in the plant. Although the patent mentions that similar results can be obtained by up- or down-regulation of the endogenous genes, one would expect that, due to the large number of plant genes, the deletion or over-expression of one of those genes has only a limited effect on the T6P concentration, if any effect at all. This is especially true for the class II TPS genes, where both a synthase-like domain and a phosphatase-like domain are present. If both domains are active, trehalose, rather than T6P, would be the end product. Moreover, for at least two Arabidopsis class II TPS genes, AtTPS7 and AtTPS8, no synthase nor phosphatase activity could be detected (Vogel et al., 2001; Eastmond et al., 2003), implying that a manipulation of these genes would not affect the T6P content of the plant at all.
Surprisingly, we found that a plant class II TPS can be used to modulate plant growth and biomass yield. Indeed, contrary to what would be expected on the basis of the literature, inactivation of plant TPS activity leads to increased stem and root growth, and increased plant biomass.